The present disclosure relates to a hydrometallurgical method for the recovery of platinum and other precious metals from complex feedstocks containing rhenium. One example of a material containing both platinum and rhenium is a class of materials called superalloys. Creep-resistant rhenium-containing nickel and cobalt base superalloys were developed to provide high temperature performance in severe environments, such as those encountered in gas turbine engines and in blades for gas turbine generators. Rhenium confers creep resistance, i.e., resistance to plastic deformation at high temperatures, and corrosion resistance on the alloys.
As used herein, the term “superalloy” means an alloy, such as a nickel and/or cobalt base alloy, containing chromium and rhenium, and comprising one or more elements selected from tungsten, tantalum, zirconium, hafnium, molybdenum, yttrium, niobium, vanadium aluminum and platinum. A typical superalloy contains 0.5 to 7% by weight rhenium, along with a major proportion (50 to 60%) nickel and minor amounts of one or more of cobalt, chromium, aluminum, molybdenum, tantalum and tungsten (e. g. 2 to 10% of each). Virgin superalloy compositions typically do not include platinum, which may be present as an undesirable impurity that is limited by product specifications.
Investment cast turbine blades are typically made from directionally solidified and single crystal superalloys (e.g., Mar M 247, and CMSX-4). Superalloy turbine blades may be provided with a thermal barrier coat (TBC) made from a refractory oxide, such as platinum aluminide (for chemical or oxidative resistance), yttria-stabilized zirconia, or a mixture of refractory oxides. To achieve a greater temperature gradient (and ultimately higher performance and fuel economy) between the hot gases flowing over the surface of the turbine blade and the internal blade structure, it is also common to cast in place a network of internal cooling structures. These complex channels are produced during the investment casting process with the use of cores held in place with pinning wires made of platinum or other noble metals having sufficient dimensional stability and oxidation resistance at de-wax and casting temperatures. Nonlimiting examples of casting temperatures range from 1300-1600° C., and nonlimiting examples of de-waxing temperatures range from 700-1000° C.
When the component (e.g., turbine blade or vane) has reached the useful service limit for flight hours and/or cycles (e.g., 6000 cycles for a single crystal turbine blade), the component is removed from service, examined and either refurbished and put back into service or recycled into revert material. Due to the expensive and exacting nature of such high temperature superalloys, which often contain rare elements such as rhenium, hafnium, tungsten, and tantalum in significant proportions, superalloy scrap is frequently recycled with virgin metals to produce an alloy billet. However, some of these materials cannot be recycled back into the revert stream when their composition does not meet stringent specifications set by the manufacturers (e.g., because they are contaminated, in some instances with excessive platinum). These end-of-life scrap materials are more valuable for their intrinsic composition and are therefore recycled for elemental recovery, and in particular, rhenium recovery.
In some instances, the platinum contained in and on superalloy components such as turbine blades, gates, sprues, rises, and the like may have a higher value than the other elements in the scrap alloy. The source of the platinum present in the alloy may be the pinning wire used in the casting process, which may result in an alloy containing a high amount of platinum. The amount of platinum in such alloys is not limited and may be as much as, for example, 0.34 wt %, 0.5 wt %, 1 wt % or higher of platinum. Platinum can also be introduced into the alloy via the remelting of platinum-aluminide coated airfoils. Standard industry practice is to abrasive shot peen the end-of-life airfoils with cut wire or other abrasive media to remove surface contamination. The present inventors have discovered that such treatment essentially welds and further diffuses the platinum into the base nickel alloy matrix, as locally high temperatures and pressures are formed at the collision site on the article.
Rhenium is present in these alloys and is conventionally recovered from these alloys in one of two ways—either by high temperature roasting of atomized superalloy in air or oxygen to remove rhenium as a volatile oxide, Re2O7, or by hydrometallurgical electrochemical and/or chemical digestion wherein the superalloy is completely or partially dissolved in an acidic aqueous solution depending upon the dissolution parameters. In high temperature roasting processes, platinum, if present, reports to the calcined base metal oxides and may be recovered through traditional hydrometallurgical approaches. In electrochemical or chemical digestion methods, aggressive conditions using highly acidic solutions and oxidizing agents may solubilize all or a part of the platinum contained in the alloy feedstock. As a consequence, platinum enters the rhenium recovery stream.
The rhenium in these alloys is recovered by loading into organic solvents or upon a variety of resins, most of which are weak- or strong base functionalized. Due to the similarity in chemical properties between rhenium and platinum, all or a part of the platinum follows the rhenium throughout the process. Platinum competes for loading with weak and strong base resins and decreases the effective sorption capacity of ion exchangers thereby contaminating the resin-eluted rhenium concentrate or strip liquor with platinum.
Superalloys are typically recycled to recover rhenium from the alloy composition, but conventional rhenium recycling processes do not attempt to separate and recover platinum present in minor amounts from the desired rhenium component because prior to the present invention, it was not known how to do so.
To this end, U.S. Patent Application Publication No. 2010/0126673 to Dasan et al. and U.S. Pat. No. 5,776,329 relate to the roasting process for removing rhenium from an alloy. WO2014158043 to Stroganov relates to a hydrometallurgical approach to removing rhenium from an alloy. Other conventional methods for removing rhenium from an alloy are disclosed in, for example, U.S. Pat. No. 8,956,582 to Feron et al.; U.S. Patent Application Publication No. 2003/0136685 to Stoller et al.; U.S. Patent Application Publication No. 2013/0078166 and U.S. Patent Application Publication No. 2011/0229366 to Luderitz et al; and U.S. Patent Application Publication No. 2009/0255372 to Olbrich. The contents of each of the above documents are incorporated herein by reference in their entirety. The conventional processes in the art, such as those above, suffer from the problem that they do not provide a method for easily separating small amounts of platinum that may be presented in the recycled alloy, in a form that is easily assayed.
Because such conventional hydrometallurgical recycling processes do not separate rhenium from any platinum that may be present in recycled superalloy compositions, rhenium pellets produced from hydrometallurgical recovery, which are consumed in the aerospace industry, often contain significant amounts of platinum impurity. A practical and commercially viable method for the extraction of platinum from rhenium-containing superalloys would be highly desirable.